It's not that they can't. They don't because you're forced to engage with them and it gives them an opportunity to get a salesperson to give you the whole sales pitch and possibly upsell in the process.
Imagine if Facebook offered RSS feeds. That’d be nice right? It wouldn’t ruin anyone’s experience if they started supporting an open standard like that.
Remember XMPP? How Google embraced that open protocol? Remember they extended it and practically turned it into their own standard? Remember how it was extinguished after they decided they could just make their own standard?
That's what people are afraid will happen with the metaverse. Facebook isn't interested in making ActivityPub better. Their only goal is to turn it into a money stream without regard to whether it survives or not. The only way to slow that down is to not play woth them.
Yeah it's been a while since I watched that series.